The Shack-Hartmann sensor is one of the most popular wavefront sensors presently available. The sensor measures the slope data of the wavefront by comparing the coordinates of Hartmann grid points from the measurement beam with coordinates from the reference beam. FIG. 1 is a schematic layout of a prior art Shack-Hartmann sensor. Hartmann invented the Hartmann test in 1900, and Roland Shack improved this technique by introducing a lenslet array in 1971. The reference beam was generated by a pin-hole light source and injected into this system as ideal wavefront to calibrate the systematic error. In FIG. 1, reference beam 105 is represented by a dashed line, and the continuous line represents the measurement beam 120. The wavefront is first collimated by the collimation lens then passes through a lenslet array 110. The Hartmann screen divides the wavefront into many sub-apertures, then the micro-lens array 110 focus the wavefront of each sub-aperture into a group of Hartmann grid points. Comparing the coordinate differences between the measured wavefront and the reference wavefront, the wavefront slopes can be calculated according to the following formula:
                                                                        ∂                W                                            ∂                x                                      ⁢                          |              i                                =                                                    x                i                mea                            -                              x                i                ref                                      f                          ⁢                                  ⁢                                                                              ∂                  W                                                  ∂                  y                                            ⁢                              |                i                                      =                                                            y                  i                  mea                                -                                  y                  i                  ref                                            f                                ,                                    (        1        )            where (xiref, yiref) (i=1, 2, . . . , m, m=t×t is the total number of grid points) is the Hartmann grid coordinates of the reference beam, (ximea, yimea) is the Hartmann grid coordinates of the measurement beam, and f is the focal length of the lenslet array.
Compared to the Hartmann test, the Shack-Hartmann wavefront sensor provides improved photon efficiency because the position of the focal spot is proportional to the average wavefront slope over each sub-aperture and the position is independent of higher-order aberrations and intensity profile variations. The Shack-Hartmann sensor is a parallel wavefront sensor operating in real time. It has application in active/adaptive optics, optical testing, opthalmology, telescope image analysis and atmosphere and random media characterizations.
However, the usage of the Shack-Hartmann sensor is restrictive because it requires an external reference, or a reference beam, and is sensitive to vibration, tilt and whole body movement. Therefore, a need exists for a sensor that provides the benefits of the Shack-Hartmann sensor without the limitations.